Declare field 'tv_nsec' of structure 'timespec' as 'i32' for AIX

[xingxue-ibm]

On AIX, <sys/stat.h> declares field tv_nsec of struct timespec as long, but the underlying system calls return a 4-byte value in both 32-bit and 64-bit modes. This PR changes to declare tv_nsec as i32 in the libc crate to avoid including the other undefined 4 bytes in the 64-bit mode.

Description

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[xingxue-ibm]

@rustbot label +stable-nominated